I think buying a costume is perfectly fine! I encourage making your own costume to get the details and fitting just how you like them, but buying a cosplay is legitimate. lol The main point is to have fun and feel good about your cosplay!

It's exactly as Roni said, while making your own cosplay is best (Because of fitment and you're able to design it exactly how you want it), there is no harm in buying cosplays already made. While I create my own costumes and props, I have also purchased a few premade cosplays in the past as well. The absolute #1 thing about cosplaying, is to have fun. It doesn't matter if you made the costume or someone else, as long as you have fun wearing it